Method for the production of photopolymerizable, cylindrical, continuous seamless flexographic printing elements, and use thereof for the production of cylindrical flexographic printing forms

ABSTRACT

Photopolymerizable cylindrical, continuously seamless flexographic printing elements are produced by applying a layer of a photopolymerizable material to the outer surface of a hollow cylinder and joining the edges by calendering. Flexographic printing elements produced in this manner are used for the production of flexographic printing plates.

The present invention relates to a process for the production ofphotopolymerizable cylindrical, continuously seamless flexographicprinting elements by applying a layer of a photopolymerizable materialto the outer surface of a hollow cylinder and joining the edges bycalendering. The present invention furthermore relates to the use offlexographic printing elements produced in this manner for theproduction of flexographic printing plates.

Cylindrical flexographic printing plates are known in principle. In thecase of a cylindrical flexographic printing plate, the printing cylinderof the printing press is provided over the entire circumference with aprinting layer or a printing relief. Cylindrical printing plates arevery important for the printing of continuous patterns and are used, forexample, for the printing of wallpapers, decorative papers orgift-wrapping papers.

In principle, the actual printing cylinder of the printing press canitself be completely surrounded by a printing layer. However, thisprocedure has the disadvantage that, on changing the printing plate, itmay be necessary to replace the entire printing cylinder. This isextremely complicated and accordingly expensive.

The use of sleeves is therefore usual. Sleeves constitute a cylindricalhollow body which has been provided with a printing layer or a printingrelief. Sleeve technology permits very rapid and simple changing of theprinting plate. The internal diameter of the sleeves corresponds to theexternal diameter of the printing cylinder so that the sleeves can besimply pushed over the printing cylinder of the printing press. Thepushing on and pushing off of the sleeves functions according to the aircushion principle: for sleeve technology, the printing press is equippedwith a special printing cylinder, i.e. an air cylinder. The air cylinderhas a compressed air connection to the end face, by means of whichcompressed air can be passed into the interior of the cylinder. Fromthere, it can emerge again via holes arranged on the outside of thecylinder. For mounting a sleeve, compressed air is passed into the aircylinder and emerges again at the outlet holes. The sleeve can then bepushed onto the air cylinder because it expands slightly under theinfluence of the air cushion, and the air cushion substantially reducesthe friction. When the compressed air feed is terminated, the expansionis eliminated and the sleeve rests firmly on the surface of the aircylinder.

Further details of the sleeve technology are disclosed, for example, in“Technik des Flexodrucks”, page 37 et seq., Coating Verlag, St. Gallen,1999.

However, high-quality round printing plates cannot be produced by simplycompletely surrounding the printing cylinder or sleeve with aflexographic printing plate processed ready for printing. In fact, afine gap which always also cuts through printing parts of the plate in atruly continuous subject remains at the abutting ends of the printingplate. This gap leads to a clearly visible line in the printed image. Inorder to avoid this line, only nonprinting depressions may be present inthis area. It is therefore not possible to print any desired patterns.Moreover, there is in this technology the danger that the solventcontained in the printing ink will penetrate into the gap and may detachthe ends of the printing plate from the printing cylinder. This leads toeven greater defects in the printed image. Even when the ends areadhesively bonded, clearly visible traces still remain in the printedimage.

For the production of high-quality round printing plates, it istherefore necessary to provide the printing cylinder or sleeve by meansof suitable techniques with a completely surrounding, relief-forming,photopolymerizable layer. This can be effected, for example, by coatingfrom solution or by ring extrusion. However, both techniques areextremely complicated and therefore correspondingly expensive. A widelyused method is therefore to wrap the printing cylinder or the sleevewith a prefabricated, thermoplastically processible layer ofphotopolymerizable material and to close the abutting edges of thephotopolymerizable layer, also referred to as the seam, as well aspossible by means of suitable techniques. Only in a second step is thecylindrical photopolymerizable flexographic printing element processedto give the final round printing plate. Apparatuses for the processingof cylindrical flexographic printing elements are commerciallyavailable.

In the production of photopolymerizable flexographic printing elementsusing prefabricated layers, it is particularly important to close theseam completely and with extreme precision. The importance of thisprocess step has further increased in recent years. Modernphotopolymerizable flexographic printing elements, for example digitallyimagable flexographic printing elements, permit the production offlexographic printing plates having substantially higher resolution thanwas previously the case. Flexographic printing is therefore alsoincreasingly being used in those areas which were previously thepreserve of other printing methods. At relatively high resolution,however, defects in the printing surface of the flexographic printingplate are also more quickly visible. For the same reason, high precisionmust also be ensured when applying the photopolymerizable,relief-forming layer. Thickness differences in the relief-forming layerhave a considerable adverse effect on the truth of running of theprinting cylinder and hence the print quality. In the case of ahigh-quality flexographic printing plate, the thickness tolerance shouldusually be not more than ±10 μm.

If the thickness tolerance of the photopolymerizable layer of the sleeveis insufficient, the surface of the sleeve must be reworked. DE-A 31 25564 and EP-A 469 375 disclose processes for improving the print quality,in which the surface of the cylindrical flexographic printing element isfirst ground and then smoothed with a suitable solvent and remainingirregularities are, if necessary, filled with binder or with thematerial of the photosensitive layer. Such a procedure is of courseextremely complicated, tedious and expensive.

Photopolymerizable, cylindrical flexographic printing elements can beproduced, for example, by applying a layer of photopolymerizablematerial to a sleeve so that the cut edges abut one another and thenheating to about 160° C. until the material begins to melt and the cutedges run into one another.

DE-A 29 11 980 discloses a process in which a printing cylinder iswrapped with a photosensitive resin film without there being asubstantial distance or a substantial overlap between the plate ends.The seam is closed by bringing the printing cylinder into contact with arotating calendar roll and joining the cut edges to one another bymelting.

During the melting of the photopolymerizable layer, however, it isscarcely possible to prevent the thickness of the photosensitive layerfrom changing in an irregular manner. The printing cylinders or sleevesproduced with the aid of such melting processes must therefore bereground and smoothed in order to obtain a good surface and to ensureprinting of high quality. EP-A 469 375 already points this out.Moreover, readily volatile components of the layer, e.g. monomers, mayevaporate during the melting of the layer, with the result that theproperties of the layer change in a disadvantageous manner.

DE 27 22 896 has proposed adhesively bonding a commercial, sheet-likephotopolymerizable flexographic printing element, together with thesubstrate film, to a printing cylinder or sleeve so that the cut edgesabut one another. The cut edges are straight and are then welded to oneanother under pressure and heat. The welding can also be effected withthe aid of a heated calendar roll by rotating the printing cylinderunder pressure in contact with the calender roll until the ends join toone another. The use of a plate having a substrate film is, however,extremely problematic. Typical substrate films have a thickness of from0.1 to 0.25 mm. If the substrate film does not completely cover thecircumference and also leaves only a minimum gap between the ends owingto a small mounting or trimming error, the empty space present betweenthe film ends fills with polymeric material during calendering, and animpression of this gap remains on the surface of the photopolymerizablelayer and leads to visible defects in the print. Such a flexographicprinting element, too, must therefore generally be reground andsmoothed.

Another technique has been proposed by U.S. Pat. No. 6,326,124, namelyclosing an existing gap with a gap-sealing compound comprising binder,UV absorber and solvent. The gap-sealing compound is, however, notidentical to the photopolymeric mixture, so that the closed gap hasproperties differing from those of the remaining relief layer, interalia different ink acceptance behavior. The gap is therefore stilldetectable in the printed image, and the printing plate is not a trulycontinuously seamless printing plate.

U.S. Pat. No. 5,916,403 proposes an apparatus of complicated design, bymeans of which a sleeve can be coated with molten photopolymer materialand the layer can be calendered. It is also possible to use plate-likepolymer material in molten or solid form for coating the sleeve. If aplate-like material is used, either a gap is left between the ends,which has to be closed by calendering at elevated temperatures, or theends overlap and the excess part likewise has to be smoothed bycalendering.

In addition to the problem of high-quality seam closure and theobtaining of a layer thickness as constant as possible, preexposure fromthe back represents a further problem of the sleeve technology. Beforethe actual main exposure, flexographic printing elements are usuallypreexposed from the back through the substrate film for a short time. Asa result of this, the relief base is prepolymerized and a bettershoulder shape, in particular of fine relief elements, in the reliefbase is achieved.

In the case of sleeves, preexposure from the back is not usuallypossible since the conventional sleeve materials, for example glassfiber-reinforced plastic or metal, are not transparent to UV radiation.EP-A 766 142 has proposed the use of transparent sleeves, in particularsleeves of polyesters, such as PET or PEN, in a thickness of from 0.25mm to 5 cm. However, these are expensive. Furthermore, special exposureunits are required for uniform exposure of the sleeve from the inside.In addition, in the case of transparent sleeves, a person skilled in theart is faced with a typical dilemma. The mechanical stability of thesleeve increases with increasing thickness of the sleeve, whereas thetransparency of the sleeve to actinic light decreases with increasingthickness of the sleeve. The problem of efficient exposure of sleevesfrom the back without a reduction in the stability of the sleeve remainsunsolved.

It is in principle possible to preexpose a solid photopolymerizablelayer from the back even before application to the sleeve. However, ithas not been possible to date for sleeves preexposed in this manner tobe welded as satisfactorily as would be expedient and necessary forproducing high-quality continuously seamless printing plates, because,as is known, only the uncrosslinked, but not the exposed, crosslinked,polymer layer can be satisfactorily welded. Furthermore, the effect ofthe preexposure is frequently lost again through the welding of thelayer ends at elevated temperatures. This leads to fine relief dots inparticular having a poor shoulder shape.

In solving this problem, DE-A 37 04 694 has therefore proposed firstapplying a first layer of photopolymer material to a sleeve, welding theseam and then polymerizing the photopolymeric layer from the front byexposure to light. In a second process step, a photopolymeric layer isapplied to the first, crosslinked layer and its seam too is welded. Thistwo-stage process is, however, very inconvenient and expensive.

It is an object of the present invention to provide an improved processfor the production of cylindrical, continuously seamless,photopolymerizable flexographic printing elements which ensures betterclosure of the seam than in the case of the known technologies and verygood truth of running. Preexposure from the back should be possible in asimple manner without adversely affecting satisfactory closure of theseam. Furthermore, reworking of the flexographic printing element bygrinding and smoothing should be avoided, and the process should becapable of being carried out as rapidly as possible. In addition, reuseof the used sleeve should be possible without great effort.

We have found that this object is achieved by a process for theproduction of photopolymerizable cylindrical, continuously seamlessflexographic printing elements by applying a layer of aphotopolymerizable material, comprising at least one elastomeric binder,ethylenically unsaturated monomers and a photoinitiator, to the outersurface of a hollow cylinder and joining the ends of the layer bycalendering, wherein the process comprises the following steps:

-   -   (a) providing a laminate at least comprising a layer of a        photopolymerizable material and a substrate film which can be        peeled off from the layer,    -   (b) cutting to size those edges of the laminate which are to be        joined, by means of miter cuts,    -   (c) pushing and locking the hollow cylinder onto a rotatably        mounted support cylinder,    -   (d) applying an adhesion-promoting layer to the outer surface of        the hollow cylinder,    -   (e) applying the laminate cut to size, on the side facing away        from the temporary substrate film, to the hollow cylinder        provided with the adhesion-promoting layer, the ends provided        with the miter cut lying substantially one on the other, but not        overlapping,    -   (f) peeling off the substrate film from the layer of        photopolymerizable material,    -   (g) joining the cut edges at a temperature below the melting        point of the photopolymerizable layer by bringing the surface of        the photopolymerizable layer on the hollow cylinder into contact        with a rotating calender roll until the cut edges have been        joined to one another,    -   (h) removing the processed hollow cylinder from the support        cylinder.

In a preferred embodiment of the invention, the adhesion-promoting layeris a double-sided adhesive tape.

We have furthermore found cylindrical continuously seamlessphotopolymerizable flexographic printing elements which are obtainableby the process described, and the use thereof for the production offlexographic printing plates by means of laser engraving or digitalimaging.

We have moreover found an apparatus particularly suitable for carryingout the process.

By means of the novel process, it is possible, in a surprisingly simplemanner, to obtain cylindrical, continuously seamless photopolymerizableflexographic printing elements of high quality. Very good seam closureis achieved. Reworking of the resulting flexographic printing element bycomplicated grinding and smoothing processes is superfluous. Preexposureof the flexographic printing element from the back is possible evenwithout it being necessary to use a transparent sleeve. It was alsosurprising and unexpected for a person skilled in the art that, in spiteof the preexposure from the back, stable and high-quality seam closureis still possible by means of the novel process. Ready-to-useflexographic printing elements can be produced within not more than 1hour from the starting materials by means of the novel process.

LIST OF FIGURES

FIG. 1: Cross section through a flexographic printing element preparedfor calendering, in which the edges to be joined have been cut to sizeby means of a miter cut and laid one on top of the other (schematic).

FIG. 2: Cross section through the preferred apparatus for carrying outthe novel process (schematic).

Regarding the present invention, the following may be statedspecifically:

For carrying out the novel process, a laminate which comprises at leastone layer of the photopolymerizable material and a substrate film whichcan be peeled off from the layer is first provided in step (a). Thelaminate can optionally also comprise a further peelable film on thatside of the layer which faces away from the substrate film. For betterpeelability, both the substrate film and the second film can be treatedin a suitable manner, for example by siliconization or by coating with asuitable nontacky release layer. Such nontacky release layers mayconsist, for example, of polyamides or polyvinyl alcohols.

The photopolymerizable material is a conventional photopolymerizablematerial which is typical for use in flexographic printing elements andcomprises at least one elastomeric binder, ethylenically unsaturatedmonomers and a photoinitiator or a photoinitiator system. Such mixturesare disclosed, for example, in EP-A 084 851.

The elastomeric binder may be a single binder or a mixture of differentbinders. Examples of suitable binders are the known vinylaromatic/dienecopolymers or block copolymers, such as conventional block copolymers ofthe styrene/butadiene or styrene/isoprene type, and furthermorediene/acrylonitrile copolymers, ethylene/propylene/diene copolymers ordiene/acrylate/acrylic acid copolymers. Of course, mixtures of differentbinders may also be used.

The binders or binder mixtures used for the novel process are preferablythose which have very little tack. Thermoplastic elastomeric binders ofthe styrene/butadiene type have proven particularly useful for the novelprocess. These may be two-block copolymers, three-block copolymers ormultiblock copolymers in which in each case a plurality of styrene andbutadiene blocks follow one another alternately. They may be linear,branched or star block copolymers. The block copolymers used accordingto the invention are preferably styrene/butadiene/styrene three-blockcopolymers, it being necessary to take into account the fact thatcommercial three-block copolymers usually comprise a certain proportionof two-block copolymers. Such SBS block copolymers are commerciallyavailable, for example under the name Kraton®. Of course, mixtures ofdifferent SBS block copolymers may also be used. A person skilled in theart makes a suitable choice from the various types depending on thedesired properties of the layer.

Styrene/butadiene block copolymers which have an average molecularweight M_(W) (weight average) of from 100 000 to 250 000 g/mol arepreferably used. The preferred styrene content of such styrene/butadieneblock copolymers is from 20 to 40% by weight, based on the binder.

The ethylenically unsaturated monomers are in particular acrylates ormethacrylates of mono- or polyfunctional alcohols, acrylamides ormethacrylamides, vinyl ethers or vinyl esters. Examples include butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, butanediol di(meth)acrylateand hexanediol di(meth)acrylate. Mixtures of different monomers can ofcourse also be used. Suitable initiators for the photopolymerization arearomatic compounds, for example keto compounds, such as benzoin orbenzoin derivatives.

The photopolymerizable mixtures may furthermore comprise conventionalassistants, for example thermal polymerization inhibitors, plasticizers,dyes, pigments, photochromic additives, antioxidants, antiozonants orextrusion assistants.

The type and amount of the components of the photopolymerizable layerare determined by a person skilled in the art according to the desiredproperties and the desired purpose of the novel flexographic printingelement.

If the flexographic printing element is to be processed by means ofdirect laser engraving to give a flexographic printing plate, the personskilled in the art can also choose formulations for the layer which areparticularly adapted to direct laser engraving. Such formulations aredisclosed, for example, by WO 02/76739, WO 02/83418, WO 03/45693 or thestill unpublished documents having the application numbers DE 102 27188.7 and DE 102 27 189.5, which are hereby incorporated by reference.

The laminates can be produced in a manner known in principle, bydissolving all components of the photopolymerizable layer in a suitablesolvent, casting the solution on the peelable substrate film andallowing the solvent to evaporate. Preferably, the laminate is produced,likewise in a manner known in principle, by melt extrusion andcalendering between the peelable substrate film and a further peelablefilm. Such photopolymerizable laminates are also commercially available,for example as nyloflex® SL (BASF Drucksysteme GmbH). Laminates whichhave two or more photopolymerizable layers can also be used. Thethickness of the laminate is as a rule from 0.4 to 7 mm, preferably from0.5 to 4 mm, particularly preferably from 0.7 to 2.5 mm.

The photopolymerizable layer can optionally be preexposed to actiniclight from the back before application to the hollow cylinder in processstep (e). The preexposure is carried out on that side of thephotopolymerizable layer which faces away from the substrate film, i.e.the subsequent underside of the layer. During the preexposure, thesurface of the photopolymerizable layer can be directly irradiated. If asecond peelable film is present, this second film can either be peeledoff or exposure is preferably effected through the film, provided thatthe film is sufficiently transparent.

The preexposure is carried out similarly to the conventional preexposureof flexographic printing plates from the back. The preexposure time isas a rule from only a few seconds to not more than one minute and isestablished by a person skilled in the art according to the desiredproperties of the layer. Of course, the preexposure time also depends onthe intensity of the actinic light. Only the layer base is polymerizedand on no account is the entire layer completely polymerized.

Depending on the desired purpose of the flexographic printing element, aperson skilled in the art determines whether a preexposure step iscarried out or not. If the further processing of the flexographicprinting element to give flexographic printing plates is intended to beeffected by a conventional method by imagewise exposure and developmentby means of a solvent, a preexposure is as a rule advisable, althoughnot always absolutely essential. If further processing by means ofdirect laser engraving is intended, a preexposure step is as a rulesuperfluous.

The preexposure should as a rule be carried out before the laminate iscut to size in step (b), in order to ensure trouble-free joining of thecut edges. If a transparent sleeve is used, the preexposure can ofcourse also be carried out from the inside of the sleeve just afterapplication of the layer to the sleeve.

In process step (b), the edges of the prepared laminate to be joined arecut to size. According to the invention, the cutting to size is carriedout by means of miter cuts, i.e. by means of cuts which are made notperpendicularly through the laminate but obliquely. The length of thelaminate is determined by the cuts so that the circumference of thesleeve can be completely surrounded and the ends provided with the mitercuts lie substantially one on top of the other but do not overlap.

As a rule, the miter angle is from 10° to 80°, preferably from 20° to70°, particularly preferably from 30° to 60°, for example 50°. Saidangles are relative in each case to the perpendicular through the layer.Both cut edges can be cut with the same miter angle. Relatively smalldeviations with the miter angle of the two cut edges from one anotherare, however, also possible without the proper joining of the cut edgesbeing adversely affected. Rather, the fact that the internal diameter ofthe photopolymerizable layer is slightly smaller than the externaldiameter can be taken into account in a particularly elegant manner byslightly different miter angles. The miter angles are calculated sothat, after cutting, the subsequent inside of the photopolymerizablelayer is exactly the correct amount shorter than the subsequent outside.However, the angles should as a rule deviate from one another by notmore than about 20°, preferably not more than 10°.

Of course, the lateral edges can also be cut to size if the width of theraw material does not fit. The lateral edges are preferably cutstraight. The width of the laminate cannot of course exceed the maximumsleeve length. As a rule, the entire length of the sleeve is not coveredwith the photopolymeric material, but in each case a narrow strip isleft uncovered at the ends. This is determined by a person skilled inthe art according to the desired properties of the flexographic printingelement.

The hollow cylinders used as supports are conventional hollow cylinderswhich are suitable for mounting on air cylinders, i.e. are capable ofexpanding slightly under the influence of compressed air. Such hollowcylinders are also referred to as sleeves, basic sleeves or the like.For the purposes of this invention, the hollow cylinders as such whichare used as supports are to be referred to below as basic sleeve whilethe term sleeve is to be reserved for the flexographic printing elementas a whole, i.e. including the photopolymerizable layer, adhesive layerand any further layers.

Basic sleeves of polymeric materials, for example polyurethanes,polyesters or polyamides, are particularly suitable for carrying out thenovel process. The polymeric materials may also be reinforced, forexample with glass fiber fabrics. They may also be multilayer materials.Furthermore, basic sleeves comprising metals, for example thosecomprising nickel, may of course be used.

The thickness, diameter and length of the basic sleeve are determined bya person skilled in the art according to the desired properties and thedesired purpose. By varying the wall thickness at constant internaldiameter (necessary for mounting on certain printing cylinders), theouter circumference of the basic sleeve and hence the printing lengthcan be determined. By printing length, a person skilled in the artunderstands the length of the printed subject on one revolution of theprinting cylinder. Suitable basic sleeves having wall thicknesses offrom 1 to 100 mm are commercially available, for example as Blue Lightfrom Rotec or from Polywest or Rossini. They may be both compressiblebasic sleeves and hard-coated basic sleeves.

For carrying out the novel process, the hollow cylinders used are pushedand locked onto a rotatably mounted support cylinder in process step (c)so that the hollow cylinder is firmly connected to the support cylinderand no movement relative to one another is possible. The supportcylinder provides firm retention for the subsequent calendering process.The locking can be effected, for example, by clamping or screwing.However, the support cylinder is preferably an air cylinder whose modeof operation corresponds to the air cylinders used in printing presses.The basic sleeve is then mounted in a very elegant manner by connectingthe air cylinder to compressed air for pushing on and hence enabling thebasic sleeve to be pushed on. After the compressed air has been switchedoff, the basic sleeve is firmly locked on the air cylinder. Thecircumference of the air cylinder can also be increased in a mannerknown in principle by using adapter or bridge sleeves (actually basicsleeves). It is thus possible to use basic sleeves having a largerinternal diameter, and greater printing lengths are thus also achievablewith the same air cylinder. Adapter sleeves are also commerciallyavailable (for example from Rotec).

In process step (d), an adhesion-promoting layer is applied to the outersurface of the hollow cylinder. The adhesion-promoting layer shouldimpart good adhesion even at elevated temperatures such as those whichwill prevail during the calendering process. It should in particularimpart very good shear strength so that the photopolymerizable layerdoes not slip on the surface of the hollow cylinder during thecalendering process. The adhesion-promoting layer may be a suitablemixture of adhesive-forming components which is applied to the surfaceof the hollow cylinder.

Preferably, however, the adhesion-promoting layer is a double-sidedadhesive film. Double-sided adhesive films for mounting printing platesare known and are available in various embodiments. In particular, theadhesive films may be foam-backed adhesive films which additionally havea damping foam layer.

The adhesive film should have a very high static shear strength. Thestatic shear strength is determined on the basis of DIN EN 1943. In thistest, a piece of the adhesive film having exactly defined dimensions isstuck onto a polished metal plate and pulled horizontally thereon withan exactly defined force. The time taken for the tape to move 2.5 mm onthe substrate is measured. The test can be carried out at elevatedtemperatures. The details of the test are listed in the example section.

An adhesive film which has a static shear strength of at least 3,preferably at least 10, particularly preferably at least 100, hours at70° C. is preferably used for carrying out the present invention.

If a foam-backed adhesive tape is used, an adhesive tape whose foamlayer consists of an open-cell foam, for example an open-cell PU foam,is preferably used. As a rule, a smoother surface of thephotopolymerizable layer than with the use of closed-cell foams isachieved therewith at the point at which the ends of the adhesive tapeabut.

The double-sided adhesive tape should be stuck onto the surface of thehollow cylinder in such a way that the cut edges exactly abut oneanother and substantially neither does a space remain between the endsnor do the ends overlap.

In process step (e), the photopolymerizable layer is applied to thehollow cylinder provided with the adhesion-promoting layer. For thispurpose, the laminate cut to size is applied, on the side facing awayfrom the temporary substrate film, to the hollow cylinder provided withthe adhesion-promoting layer. If a second peelable film is present,this, including any nontacky release layer present, must of course beremoved before the application. The application should be effectedwithout bubbles and is carried out in such a way that the ends providedwith the miter cut lie substantially one on top of the other but do notoverlap.

FIG. 1 schematically shows a cross section through a flexographicprinting element which has been prepared for calendering and in whichthe edges to be joined have each been cut to size by means of a mitercut and placed one on top of the other: an adhesive tape (2) and thephotopolymerizable layer (3) are applied to the basic sleeve (1). Theedges to be joined are cut to size by means of miter cuts (4) and placedone on top of the other. The arrow (7) indicates the preferred directionof rotation of the flexographic printing element during calendering. Theair cylinder has been omitted in FIG. 1 for greater clarity.

In order to ensure that the cut edges lie properly one on top of theother, the application of the layer element is expediently thereforebegun with the cut edge whose bottom is longer than the top (FIG. 1,(5)). After complete wrapping, the second cut edge (6), at which the topis longer than the bottom, finally lies on the first cut edge.

After the application of the layer element, the substrate film,including any nontacky release layer present, is peeled off from thelayer of photopolymeric material (process step (f)).

In process step (g), the cut edges are joined. For joining the cutedges, the surface of the photopolymerizable layer on the hollowcylinder is brought into contact with a rotating calender roll until thecut edges have been joined to one another. The support cylinder and thecalender roll rotate in opposite directions. The necessary calenderpressure is determined by a person skilled in the art according to thetype of photopolymerizable layer by adjusting the distance between thesupport cylinder and the calender roll. The calendering temperaturedepends on the type of photopolymerizable layer and the desiredproperties. According to the invention, however, the temperature of thecalender roll is adjusted so that the temperature of thephotopolymerizable layer is in any case below its melting point, so thatthe adverse effects mentioned at the outset and due to melting of thelayer are avoided.

Expediently, heat is supplied by using a calender roll heated from theinside. However, heat can also be supplied, for example, by IR lamps orwarm gas streams. Of course, heat sources can also be combined. As arule, the temperature during calendering is from 80 to 130° C.,preferably from 90 to 120° C., measured in each case at the surface ofthe photopolymerizable layer.

The calendering is particularly preferably carried out in such a waythat the coated hollow cylinder rotates in the direction (7) duringcalendering. The preferred direction of rotation is indicated in FIG. 1and FIG. 2 by the arrow (7) and can be achieved by appropriateadjustment of the direction of rotation of the rolls. Since the calenderroll and the coated hollow cylinder rotate in opposite directions duringcalendering (FIG. 2), the upper cut edge (6) is calendered in thedirection of decreasing layer thickness in the case of this direction ofrotation. Consequently, opening of the gap is advantageously avoidedalthough it is also possible in special cases to calender in theopposite direction. As a rule, about 15 minutes are required forcomplete closure of the gap, this time of course also depending on thechosen temperature and the pressure.

As a result of the calendering, the cut edges are firmly joined to oneanother. Joining is effected mainly in the region of the photopolymericlayer which was not preexposed. In the lower layer region which waspreexposed, the edges do not join or at least do not join so well. Thisof course also depends on the intensity of the preexposure and hence onthe degree of the preliminary crosslinking. By means of the novelprocess, however, it is surprisingly nevertheless possible to achievevery good, durable joining of the edges.

After the closure of the seam and any cooling, the processed hollowcylinder/prepared sleeve is removed again from the support cylinder.

The apparatus shown schematically in FIG. 2 has proven very particularlyuseful for carrying out the process, without the invention being limitedthereby to the use of this apparatus.

The apparatus has an air cylinder (8) and a heatable calender roll (9).Both cylinders are rotatably mounted. The suspensions of the cylindersare not shown for the sake of clarity. At least one of the two rolls ismoreover mounted so as to be displaceable in the horizontal direction sothat the rolls can be moved together and apart. This is indicatedschematically by the double-headed arrow (13). For heating, for example,electrical heating elements can be installed in the calender roll or hotoil can flow through the roll. An auxiliary roll (1) whose distance fromthe air cylinder can be adjusted is also provided as an aid formounting. The auxiliary roll (10) is preferably arranged below the aircylinder. The auxiliary roll is preferably a rubber roll. The apparatusfurthermore has a feed apparatus (11) for the photopolymerizable layerand/or the adhesive film. The feed apparatus may simply be, for example,an assembly table on which the photopolymerizable layer and/or theadhesive film can be placed and can be inserted from there uniformlyinto the gap between basic sleeve and auxiliary roll. This can beeffected manually, preferably by means of a suitable pushing apparatus.The calender roll should have very little adhesion to thephotopolymerizable layer. For example, it may be polished or may have acoating for eliminating tack, for example a Teflon coating. Theapparatus can of course also comprise further assemblies.

The operation of the apparatus is explained by way of example belowwithout there being any intention thereby to limit the invention to thismode of operation or at all to the use of the apparatus. For carryingout the process, a basic sleeve (12) is first pushed onto the aircylinder (8). Thereafter, the adhesive film is cut to size on theassembly table (11), the air cylinder is rotated and the film is slowlypushed into the gap between auxiliary roll (10) and the air cylinder (8)provided with the basic sleeve (12). The adhesive film is carried alongas a result of the rotation, the auxiliary roll pressing the film ontothe basic sleeve so that the adhesive film firmly adheres to the basicsleeve without bubbles. The protective film is then peeled off from theadhesive film. The basic sleeve is then provided with anadhesion-promoting layer. In the next step, the photopolymerizablelaminate cut to size is pushed into the gap, carried along and pressedfirmly by the auxiliary roll (10). The unpreexposed or preexposedunderside of the layer faces the basic sleeve. If the photopolymerizablelayer has a second, peelable film, this is peeled off beforehand. Afterthe substrate film of the laminate has been peeled off, the calenderroll and the air cylinder provided with basic sleeve, adhesion-promotinglayer and photopolymerizable layer are brought into contact with oneanother, caused to rotate, and the gap is closed by calendering with thehot calender roll. The preferred direction of rotation duringcalendering is (7).

The process steps (a) to (h) can be carried out in this sequence.However, variations are also possible. Thus, it is entirely possiblefirst to apply the adhesion-promoting layer (step (c)) and thephotopolymerizable layer (step (e)) to the basic sleeve and onlythereafter to push the coated basic sleeve onto the support cylinder(c).

The cylindrical, continuously seamless flexographic printing elementsobtainable by the novel process differ from those known from the priorart. Traces of the miter cut are still detectable as a discontinuity inthe region of the closed seam by means of suitable analytical methods(for example microscopic observation, if necessary by means of polarizedlight). If preexposure was effected, the seam is clearly detectable inthe lower layer region. Nevertheless, a printing layer completelyuniform with regard to the printing properties is obtained, so that avisible seam is no longer present in the printed image. Stress-strainmeasurements using layer samples from the region of the closed seam andthose without a seam have comparable values.

In the novel process, no monomers at all evaporate during calendering,owing to the comparatively low temperature. Furthermore, the effect ofthe preexposure from the back is also retained. Both contribute toward aconstant high layer quality, a requirement for high-quality printingplates.

The novel flexographic printing elements are very useful as a startingmaterial for the production of cylindrical, continuously seamlessflexographic printing plates.

The further processing to give flexographic printing plates can becarried out by various techniques. The flexographic printing elementscan, for example, be exposed imagewise in a manner known in principleand the unexposed parts of the relief-forming layer then removed bymeans of a suitable development process. The imagewise exposure can becarried out in principle by surrounding the sleeve with a photographicmask and effecting exposure through the mask.

Preferably, however, the imaging is carried out by means of digitalmasks. Such masks are also known as in situ masks. For this purpose, adigitally imagable layer is first applied to the photopolymerizablelayer of the sleeve.

The digitally imagable layer is preferably a layer selected from thegroup consisting of IR-ablative layers, inkjet layers andthermographically inscribable layers.

IR-ablative layers or masks are opaque to the wavelength of actiniclight and usually comprise a binder and at least one IR absorber, forexample carbon black. Carbon black also ensures that the layer isopaque. In the IR-ablative layer, a mask can be inscribed by means of anIR laser, i.e. the layer is decomposed and material removed in the areasin which the laser beam is incident on said layer. Examples of theimaging of flexographic printing elements using IR-ablative masks aredisclosed, for example, in EP-A 654 150 or EP-A 1 069 475.

In the case of inkjet layers, a layer inscribable using inkjet inks andtransparent to actinic light, for example a gelatin layer, is applied. Amask is applied to said gelatin layer using opaque ink by means ofinkjet printers. Examples are disclosed in EP-A 1 072 953.

Thermographic layers are layers which contain substances which becomeblack under the influence of heat. Such layers comprise, for example, abinder and an organic silver salt and can be imaged by means of aprinter having a thermal printing head. Examples are disclosed in EP-A 1070 989.

The digitally imagable layers can be produced by dissolving ordispersing all components of the respective layer in a suitable solventand applying the solution to the photopolymerizable layer of thecylindrical flexographic printing element, followed by evaporation ofthe solvent. The application of the digitally imagable layer can beeffected, for example, by spraying on or by means of the techniquedescribed by EP-A 1 158 365. Components soluble in water orpredominantly aqueous solvent mixtures are preferably used for theproduction of the digitally imagable layer.

After the application of the digitally imagable layer, the latter isimaged by means of the respective suitable technique and then the sleeveis exposed to actinic light through the mask formed in a manner known inprinciple. In particular, UVA or UV/VIS radiation is known to besuitable as actinic, i.e. chemically active, light. Rotary, cylindricalexposure units for uniform exposure of sleeves are commerciallyavailable.

The development of the imagewise exposed layer can be carried out in aconventional manner by means of a solvent or of a solvent mixture. Theunexposed parts of the relief layer, i.e. those parts covered by themask, are removed by dissolution in the developer, while the exposed,i.e. the crosslinked, parts are retained. The mask or the remainder ofthe mask is likewise removed by the developer if the components aresoluble therein. If the mask is not soluble in the developer, it is, ifrequired, removed with the aid of a second solvent before development.

The development can also be effected thermally. In the thermaldevelopment, no solvent is used. Instead, the relief-forming layer isbrought into contact with an absorbing material after the imagewiseexposure and is heated. The absorbing material is, for example, a porousnonwoven, for example comprising nylon, polyester, cellulose orinorganic materials. It is heated to a temperature such that theunpolymerized parts of the relief-forming layer liquefy and can beabsorbed by the nonwoven. The saturated nonwoven is then removed.Details of the thermal development are disclosed, for example, by U.S.Pat. No. 3,264,103, U.S. Pat. No. 5,175,072, WO 96/14603 or WO 01/88615.The mask can, if necessary, be removed beforehand by means of a suitablesolvent or likewise thermally.

The production of cylindrical flexographic printing plates from thephotopolymerizable, continuously seamless flexographic printing elementscan also be carried out by means of direct laser engraving.

In this process, the photopolymerizable layer is first completelycrosslinked in the total volume by means of actinic light withoutplacing a mask on top. A printing relief is then engraved into thecrosslinked layer by means of one or more lasers.

The uniform crosslinking can be effected using conventional rotary,cylindrical exposure units for sleeves, as described above. However, itcan also particularly advantageously be effected on the basis of themethod described in WO 01/39897. Here, exposure is effected in thepresence of an inert gas which is heavier than air, for example CO₂ orAr. For this purpose, the photopolymerizable, cylindrical flexographicprinting element is lowered into an immersion tank which is filled withinert gas and whose walls are preferably lined with a reflectivematerial, for example aluminum foil. The lowering is preferably effectedin such a way that the axis of rotation of the cylindrical flexographicprinting element is vertical. The immersion tank can be filled withinert gas, for example, by introducing dry ice into the immersion tank,which displaces the atmospheric oxygen on vaporization. Exposure is theneffected from above by means of actinic light. In principle, theconventional UV or UVNIS sources of actinic light can be used for thispurpose. Radiation sources which emit substantially visible light and noUV light or only small amounts of UV light are preferably used. Lightsources which emit light having a wavelength of more than 300 nm arepreferred. For example, conventional halogen lamps can be used. Theprocess has the advantage that the ozone pollution usual in the case ofshort-wave UV lamps is virtually completely absent, safety measures toprevent strong UV radiation are as a rule unnecessary and no expensiveapparatuses are required. Thus, this process step can be carried outparticularly economically.

In direct laser engraving, the relief layer absorbs laser radiation tosuch an extent that said layer is removed or at least detached in thoseparts in which it is exposed to a laser beam of sufficient intensity.Preferably, the layer will vaporize or thermally or oxidativelydecompose before melting, so that its decomposition products are removedfrom the layer in the form of hot gases, vapors, fumes or smallparticles.

Lasers which have a wavelength of from 9 000 to 12 000 nm areparticularly suitable for engraving the relief-forming layers usedaccording to the invention. Particular examples of these are CO₂ lasers.The binders used in the relief-forming layer absorb the radiation ofsuch lasers to a sufficient extent to permit engraving.

A laser system which has only a single laser beam can be used forengraving. Preferably, however, laser systems which have two or morelaser beams are used. Preferably, at least one of the beams is speciallyadapted for producing coarse structures and at least one of the beams isspecially adapted for writing fine structures. With such systems, it ispossible to produce high-quality printing plates in a particularlyelegant manner. For example, the beam for producing the fine structuresmay have a lower power than the beams for producing coarse structures.For example, the combination of a beam having a power of from 50 to 150W with two beams of 200 W or more each has proven particularlyadvantageous. Multibeam laser systems particularly suitable for laserengraving and suitable engraving methods are known in principle and aredisclosed, for example, in EP-A 1 262 315 and EP-A 1 262 316.

The depth of the elements to be engraved depends on the total thicknessof the relief and on the type of elements to be engraved and isdetermined by a person skilled in the art according to the desiredproperties of the printing plate. The depth of the relief elements to beengraved is at least 0.03 mm, preferably 0.05 mm—the minimum depthbetween individual dots is mentioned here. Printing plates having reliefdepths which are too small are generally unsuitable for printing bymeans of the flexographic printing technique because the negativeelements fill with printing ink. Individual negative dots should usuallyhave greater depths; for those of 0.2 mm diameter, a depth of at leastfrom 0.07 to 0.08 mm is usually advisable. In the case of surfaces whichhave been removed by engraving, a depth of more than 0.15 mm, preferablymore than 0.4 mm, is advisable. The latter is of course possible only inthe case of a correspondingly thick relief.

The cylindrical flexographic printing plate obtained can advantageouslybe subsequently cleaned in a further process step after the laserengraving. In some cases, this can be effected by simply blowing offwith compressed air or brushing off. It is however preferable to use aliquid cleaning agent for the subsequent cleaning in order also to beable to remove polymer fragments completely.

For example, aqueous cleaning agents which substantially comprise waterand optionally small amounts of alcohols, and which may containassistants, such as surfactants, emulsifiers, dispersants or bases, forsupporting the cleaning process, are suitable. Water-in-oil emulsions,as disclosed by EP-A 463 016, are also suitable.

The cylindrical printing plates obtained by means of digital imaging orby means of direct laser engraving are very useful for the printing ofcontinuous patterns. They may have any desired printing areas even inthe region of the seam without the seam also being visible in theprinted image. If adhesive tape was used as the adhesion-promotinglayer, the printing layer can be very easily peeled off from the basicsleeve again and reused. In this case, it is possible to use basicsleeves of different types, for example compressible basic sleeves orhard-coated basic sleeves.

The examples which follow illustrate the invention:

Methods of Measurement:

Determination of static shear strength of the adhesive film on the basisof DIN EN 1943, Klebebänder—Messung des Scherwiderstandes unterstatischer Belastung (January 2003 edition).

Testing was carried out according to method A described. For the test, asteel plate specified in DIN EN1943 was used. The steel plate wasclamped perpendicularly in a holding apparatus. A 25 mm wide test stripof the adhesive film was stuck onto said steel plate so that the area ofcontact with the steel plate was exactly 25 mm×25 mm and a part of theadhesive tape hung perpendicularly below the steel plate. The test massof 1 kg was suspended from the freely hanging end of the adhesive tape.The test was carried out at 70° C. The time taken for the adhesive tapeto slip 2.5 mm downward on the steel plate was determined.

Provision of the Laminate:

Layer Element 1:

The following starting materials were used for the photopolymerizable,elastomeric layer: Component Amount SBS block copolymer (M_(w) 125 00055% g/mol, styrene content 30% by weight (Kraton D 1102)) Polybutadieneoil plasticizer 32% Hexanediol diacrylate monomer 10% Photoinitiator  2%Additives (heat stabilizer, dye)  1% Total 100% 

The layer element used as starting material for the novel process wasproduced from the components in a manner known in principle by meltextrusion and calendering between two peelable PET films provided with anontacky release layer (substrate film and second film). Thephotopolymerizable layer had a thickness of 1.14 mm.

Layer Element 2:

A layer element of the same type as described in the case of layerelement 1 was produced, except that the following starting materialswere used for the photopolymerizable layer. Component Amount SBS blockcopolymer (M_(w) 125 000 58% g/mol, styrene content 30% by weight,extended with about 33% of oil (Kraton D 4150)) Secondary binder SBtwo-block 10% copolymer, M_(w) 230 000 g/mol (Kraton DX 1000)Polybutadiene oil plasticizer 23% Hexanediol diacrylate monomer  7%Photoinitiator  1% Additives (heat stabilizer, dyes)  1% Total 100% 

Production of the Cylindrical, Continuously Seamless FlexographicPrinting Elements:

EXAMPLE 1

An apparatus of the type described above was used for the procedure. Theauxiliary roll (1) was rubber-coated. The calender roll was siliconized.A simple assembly table functioned as feed apparatus (11).

A basic sleeve (Blue Light, from Rotec, internal diameter 136.989 mm,external diameter 143.223 mm) was pushed onto the air cylinder of theapparatus described above and was fixed. A 500 μm thick compressibleadhesive tape having a shear strength (Rogers SA 2120, shear strength at70° C.>100 h) was then applied to the basic sleeve without a gap. Thecompressible layer of the adhesive tape consisted of an open-cell PUfoam.

Layer element 1 was exposed to actinic light from the back for 12seconds through one of the two PET films. Layer element 1 was then cutto size. The two abutting edges were trimmed with an angle of 50° and55°, relative in each case to the perpendicular, in such a way that thepreexposed side of the layer was shorter than the unpreexposed side. Thefilm on the preexposed side was peeled off, including the nontackyrelease layer, and the layer element was applied with the preexposedside, with constant rotation, to the basic sleeve provided with theadhesive film. After the application of the layer, the second PET film,including the nontacky release layer, was peeled off.

The layer was pressed firmly by means of the auxiliary roll (10). Thecalender roll was heated to about 130° C., rotated (50 rpm) and broughtinto contact with the photopolymerizable layer. The distance between thecalender roll and the air cylinder was adjusted so that a negative gapof 300 μm resulted (i.e. the calender roll was pressed 300 μm into theelastomeric, photopolymerizable layer). Calendering was effected for 15minutes for closing the gap. Rotation was effected in direction (7). Thesurface temperature of the photopolymerizable layer was about 95° C.Thereafter, the rolls were moved apart again and the coated basic sleevewas removed again from the air cylinder after cooling.

A cylindrical, photopolymerizable continuously seamless flexographicprinting element was obtained. The surface of the printing element wascompletely flat in the region of the seam and no traces of the seam atall were detectable. A cut in the region of the seam showed that theseam was not completely closed in the preexposed layer region but theclosure in the upper layer region was so good that overall an extremelydurable joint was obtained. Stress-strain measurements on the exposedmaterial showed that the samples having a gap and samples comprising thesolid surface do not differ substantially with respect to the tensilestress.

EXAMPLE 2

The procedure was as in example 1, except that layer element 2 was usedas starting material. Furthermore, no preexposure was carried out andthe calender roll was heated to 135° C. The surface temperature of theflexographic printing element during the calendering was 100° C.

A cylindrical, photopolymerizable continuously seamless flexographicprinting element was obtained.

COMPARATIVE EXAMPLE 1

The procedure was as in example 1, except that the laminate was cut tosize not by means of miter cuts but by making two perpendicular cuts.After the sleeve had been wrapped in the photopolymerizable material, asmall V-shaped gap remained at the abutment point of the twoperpendicular cuts. The gap could be closed by calendering but a smallindentation remained at the seam.

COMPARATIVE EXAMPLE 2

The procedure was as in example 1, except that the calender roll washeated to 150 to 160° C. The surface temperature of the flexographicprinting element was about 120° C. Although the seam could be closed,the surface of the photopolymerizable layer was considerably deformed bythe excessively high thermal load and had excessively large tolerancesafter cooling. Regrinding and smoothing were necessary in order toobtain a quality sufficient for flexographic printing. Furthermore, thepreexposure through the back lost its effect.

Comparative Example 3

A layer element was produced by extrusion and calendering using the samecomponents as described in the case of layer element 1. However, onlyone of the two PET films was peelable while the other PET film wasbonded to the photopolymerizable layer by a mixture ofadhesion-promoting components. Preexposure was effected through thenonpeelable PET film as described in example 1. After cutting to size asdescribed, the layer element was mounted with the preexposed side andwith the nonpeelable film on the basic sleeve and was calendered. Seamclosure was obtained but the abutment point of the nonpeelable PET filmwas still visible as an impression on the layer surface.

Comparative Example 4

The procedure was as in example 1, except that an adhesive tape having ashear strength of only 2.3 h at 70° C. was used. Although thephotopolymerizable layer could be applied without problems, the adhesivetape slipped slightly on the basic sleeve during calendering. Theabutment point of the adhesive tape was still clearly visible as animpression in the surface of the photopolymerizable layer.

Further Processing to Give Flexographic Printing Plates

EXAMPLE 3

An IR-ablative digitally imagable layer comprising carbon black and abinder was applied to the cylindrical, photopolymerizable flexographicprinting element according to experiment 1 in a manner known inprinciple by means of a ring coater as described by DE 299 02 160.

The resulting photosensitive flexographic printing element with theIR-ablative layer was then inscribed imagewise with a continuous patternby means of an Nd/YAG laser. The pattern was chosen so that printingparts in the region of the seam were also provided.

The sleeve provided with an image was exposed to actinic light in arotary, cylindrical exposure unit for 20 minutes, then developed withthe aid of a flexographic washout agent (nylosolv® II), dried for 2hours at 40° C. and postexposed for 15 minutes to UV/A and UV/C.

EXAMPLE 4

The photosensitive flexographic printing element according to experiment2 was exposed to a mercury halide lamp from Hönle under CO₂ inert gas inan immersion tank lined with aluminum foil, and the photosensitive layerwas completely crosslinked.

A printing relief was then engraved into the crosslinked relief layer bymeans of lasers using a laser system as described by EP-A 1 262 315. Acontinuous subject was engraved, in such a way that printing parts werealso present in each case in the region of the seam.

COMPARATIVE EXAMPLES 5, 6 AND 7

The procedure was as in example 3, except that in each case theflexographic printing elements according to C1, C3 and C4 were used.

Printing Experiments

Printing experiments were carried out using the cylindrical flexographicprinting plates obtained from the experiments and comparativeexperiments.

Printing press: W&H (Windmöller and Hölscher), printing speed: 150m/min, print medium: PE film

In the novel examples, a four-color proof showed no gap either in thesingle color extractions or in the overprinting of all colors, whereasthe gap was still visible in the comparative experiments.

The results are summarized in table 1. TABLE 1 Results of theexperiments and comparative experiments Flexographic Produced fromprinting plate flexographic printing No. element No. Comment Example 3Example 1 Uniform continuous subject, no gap visible in the printedimage Example 4 Example 2 Uniform continuous subject, no gap visible inthe printed image C5 Example 1 Gap was visible in the printed image C6Example 1 Gap was visible in the printed image C7 Example 1 Gap wasvisible in the printed image

Table 1: Results of the experiments and comparative experiments

For checking the printing length, a printing plate having an identicalplate on sleeve structure was processed simultaneously with the sleeveaccording to experiment 3 for the black color and proof printing waseffected using the three colors of the continuously seamless sleeves.Result: No deviation of the printing lengths of the individual colors,i.e. the sleeves thus produced experience no change in the circumferenceduring calendering and are compatible with other plate structures.

1. A process for the production of photopolymerizable cylindricalcontinuously seamless flexographic printing elements by application of alayer of a photopolymerizable material, comprising at least oneelastomeric binder, ethylenically unsaturated monomers and aphotoinitiator, to the outer surface of a hollow cylinder and joining ofthe layer ends by calendering, wherein the process comprises thefollowing steps: (a) providing a laminate of a photopolymerizablematerial a substrate film which can be peeled off from the layer, andoptionally a further peelable film on that side of the layer which facesaway from the substrate film, (b) cutting the edges of the laminate tobe joined to size by means of bevel cuts, (c) pushing the hollowcylinder onto a rotatably mounted support cylinder and locking itthereon, (d) applying an adhesion-promoting layer to the outer surfaceof the hollow cylinder, (e) applying the laminate, if applicable, afterpeeling off the further peelable film, cut to size, on the side of thephotopolymerizable layer facing away from the temporary substrate film,to the hollow cylinder provided with the adhesion-promoting layer, theends provided with the bevel cut substantially resting against oneanother but not overlapping, (f) peeling off the substrate film from thelayer of photopolymerizable material, (g) joining the cut edges at atemperature below the melting point of the photopolymerizable layer bybringing the surface of the photopolymerizable layer on the hollowcylinder into contact with a rotating calender roll until the cut edgesare joined to one another, (h) removing the processed hollow cylinderfrom the support cylinder.
 2. A process as claimed in claim 1, whereinthe adhesion-promoting layer is a double-sided adhesive film.
 3. Aprocess as claimed in claim 2, wherein the adhesion film has a staticshear strength, measured according to DIN EN 1943, of at least 3 h at70° C.
 4. A process as claimed in claim 1, wherein the layer ofphotopolymerizable material comprises a further peelable film on thatside of the layer which faces away from the substrate film, whichpeelable film is peeled off before process step (e).
 5. A process asclaimed in claim 1, wherein, before process step (e), the layer ofphotopolymerizable material is preexposed to actinic light, directly orthrough the further peelable film, from the side facing away from thesubstrate film.
 6. A process as claimed in claim 5, wherein thepreexposure is effected before process step (b).
 7. A process as claimedin claim 1, wherein the coated hollow cylinder rotates in the direction(7) during calendering.
 8. A process as claimed in claim 1, wherein thetemperature of the plate surface during calendering is from 80 to 130°C.
 9. A process as claimed in claim 1, wherein the support cylinder isan air cylinder.
 10. A process as claimed in claim 1, wherein, in afurther process step (i), a digitally imagable layer is applied to thephotopolymerizable layer.
 11. A process as claimed in claim 10, whereinthe digitally imagable layer is one selected from the group consistingof IR-ablative layers, inkjet layers and thermographically recordablelayers.
 12. A cylindrical, continuously seamless, photopolymerizableflexographic printing element obtainable by the process of claim
 1. 13.A cylindrical, continuously seamless, photopolymerizable flexographicprinting element having a digitally imagable layer and obtainable by theprocess of claim
 10. 14. The use of a digitally imagable cylindricalflexographic printing element as claimed in claim 13 for the productionof cylindrical, continuously seamless flexographic printing plates,wherein the digitally imagable layer is recorded on imagewise, thephotopolymerizable layer is exposed to actinic light through the maskformed and unexposed parts of the layer are removed in a developmentstep.
 15. The use as claimed in claim 14, wherein the development of theexposed layer is carried out by means of a solvent or solvent mixture.16. The use as claimed in claim 14, wherein the development of theexposed layer is carried out thermally.
 17. The use of a cylindricalflexographic printing element as claimed in claim 12 for the productionof cylindrical, continuously seamless flexographic printing plates,wherein the photopolymerizable layer is completely crosslinked withactinic light and a printing relief is then engraved into thepolymerized layer by means of one or more lasers.
 18. The use as claimedin claim 17, wherein the laser or lasers has or have a wavelength from 9000 to 12 000 nm.